U.S. Pat. No. 5,657,400 discloses a method far the correction of bad pixel values of an X-ray detector. A method is described for the identification and correction of defective pixels. For each pixel that is identified as being defective a correction code is stored in a defect table. For the identification a plurality of offset images is acquired first so as to be averaged. Subsequently, a plurality of X-ray images is acquired with a uniform exposure during a special calibration procedure; these images are also averaged. Said two averaged images are subtracted from one another. Defective pixels are determined on the basis of the difference. The calibration procedure is carried out prior to the actual X-ray exposure, without a patient being resent in the X-ray beam path.
The error-free reproduction of X-ray images that is required for diagnostic purposes makes it necessary to correct the image data produced by the X-ray examination apparatus. For example, the geometry of X-ray detectors induces imaging defects that are eliminated by means of correction tables during the image processing. Furthermore, the X-ray detector may involve offset values which are also corrected by way of image processing. During such image processing all imaging defects that are known to the manufacturer are corrected, the image processing then being based only on image parameters that are made available a priori by the manufacturer of the X-ray examination apparatus or on parameters that are obtained by calibration processes performed by means of such X-ray examination apparatus.
However, the range of applications of X-ray examination apparatus is very diverse, so that the loads and effects are different for every X-ray examination apparatus.
Co-operation of a plurality of components is necessary so as to generate an X-ray image, each individual component being subject to an ageing process or to unpredictable environmental influences or loads. Such unpredictable effects have a varying effect on the quality of the X-ray image to be formed.
This type of influencing can be mitigated by periodic updating of correction tables involved in the image processing.
The errors that may occur during the acquisition of X-ray images, however, are not stable in time and not independent either from the relevant mode of operation of the X-ray examination apparatus. A periodic update can be applied so as to ensure a basic quality of the X-ray images acquired by means of the X-ray examination apparatus. However, between the instants at which such a periodic update is carried out there is the risk of overcompensation or undercompensation of image defects or imaging defects and also the risk of occurrence of sudden errors during the acquisition or image processing.
The known method for eliminating imaging defects utilizes image parameters that have not been obtained during the clinical examination. The instantaneous imaging error is not evaluated and a corresponding adaptation method is not determined either.
Therefore, it is an object of the invention to provide an X-ray examination apparatus and a method for forming X-ray images in which imaging defects that are due to imperfections in the image generating and processing chain are automatically corrected.